A variety of techniques are presently available for obtaining x-ray images. One common technique employs an x-ray absorbing phosphor screen which emits optical radiation which exposes photographic film held adjacent to the phosphor screen. This technique offers the advantage of high resolution, but is not effective in real time because of the need to develop the photographic film to obtain a visible x-ray image.
Another imaging device is the x-ray image intensifier tube. In this device x-rays are absorbed by a fluorescent screen which emits photons that are in turn absorbed in a layer of photoelectron emitting material. This material emits electrons which are then accelerated and focused on a phosphor screen to produce a higher intensity visible image. While this system operates in real time, it suffers from the disadvantage that it produces relatively low resolution images as a result of optical scattering, imperfect electron optics, loss of sharpness in the optics coupling the image intensifier to the camera and other effects. In addition, it is bulky, fragile, expensive and requires high voltage to operate.
Lubowski et al. U.S. Pat. No. 4,011,454, issued Mar. 8, 1977 and entitled "Structured X-Ray Phosphor Screen," which is assigned to the assignee of the present invention and incorporated by reference for background purposes, discloses a modified x-ray image intensifier which provides increased resolution through the use of a structured scintillator material as the fluorescent screen. This structured scintillator screen is produced by a vacuum evaporation process in which cesium iodide (CsI) is evaporated from a source boat and deposited on a topographically structured surface to produce columnar scintillator elements. During the deposition, the structured surface is maintained at a temperature in the range of 50.degree. C. to 150.degree. C. The scintillator is then fired at 450.degree. C. to 500.degree. C. to compact the scintillator elements. This is followed by another firing at 450.degree. C. to 500.degree. C. to compact the scintillator. Following the final deposition, the scintillator is fired at 615.degree. C.
In recent years, the art of electronic image processing has advanced rapidly. These advances have been made computed topography (CT) machines not only feasible, but very valuable medical diagnostic tools. However, such machines are substantially larger and more expensive than typical x-ray machines and are more suitable for obtaining images of slices through the body rather than a chest x-ray type of image of the body.
There is a need for high resolution x-ray imaging systems which have an improved modulation transfer function (MTF). The modulation transfer function is the output contrast divided by the input modulation and is a function of the spatial frequency of the modulation.
Semiconductor photosensitive imaging arrays are widely available today for use in television cameras, facsimile machines and a wide variety of other applications. The device disclosed in Beerlage U.S. Pat. No. 4,906,850, issued Mar. 6, 1990, is an example of a radiographic image detecting device using a semiconductor photodetector matrix and a scintillator array. The Beerlage device discloses use of a protective layer of silicon oxide or silicon nitride disposed over the semiconductor photodetector matrix, with grooves cut to form land portions on which scintillator crystals are formed. Devices such as that disclosed by Beerlage proved problematic in that formation of uniform protective layers having both the required thickness and good optical transmission qualities to form the land portions is difficult when using silicon nitride or silicon oxide alone. Other materials, such as polyimides, can be used for protective and/or insulative layers in such a device as they exhibit good optical transmission characteristics and are readily patterned, even in thick layers. Such polymer layers present additional problems, however, as earlier-deposited layers of the polymer in a photodetector array can swell, crack, or otherwise become structurally degraded when exposed to the organic solvents necessarily present when subsequent uncured layers of polymers are deposited. These organic solvents readily leach through the desirably thin common electrode material, such as indium tin oxide, and can thereby cause structural damage to a device having an underlying polymer layer. Making the electrode thicker to resist such solvents is undesirable as it reduces the optical transparency of the electrode and increases its electrical resistance, and both of these changes adversely affect overall device performance.
There is thus a need for a readily fabricated and effective device for providing increased resolution in real time radiation imagery for x-ray images of the type typically provided on x-ray film and for an electronic signal output of the radiation image rather than optical output to facilitate electronic processing of the image data.
Accordingly, one object of the present invention is to provide an electronic output, x-ray sensing two-dimensional imaging array having high resolution. Another object of the present invention is to provide an imaging device that is readily fabricated using an organic film, such as a polymer, to form topographically patterned surface features as foundations to support individual scintillator elements disposed over a photodetector array to provide high resolution imaging signals.